Experimental investigation on multiple-site fatigue failure of the hollow compressor impeller

Abstract

This study conducts a systematic experimental investigation on multiple-site damage (MSD) in aero-engine hollow compressor impellers, focusing on MSD fatigue fracture growth and coalescence behaviors and mechanisms. Comparative analyses of literature research, standard specimen tests, and full-scale disk test experiences conclude that elevated cyclic radial stresses, especially in thin-walled disk webs, facilitate the occurrence of MSD fractures. Extending from previous numerical studies, a full-scale hollow impeller fatigue test on a high-speed spin tester revealed an over 153 mm circumferential through-wall MSD failure in the outlet web, accounting for 33 % of its circumference. Challenges in traditional nondestructive inspections for such complex structures emphasize the effectiveness of synergetic fractography analysis in investigating the MSD failure. Thousands of scanning electron microscope (SEM) images, examined at macro and micro levels with the assistance of metallography and stereomicroscopy, unfolded the MSD fracture features of crack initiation, propagation, coalescence, and interaction. Besides the well-known macroscopic rachet marks, this study is the first to identify and analyze various microscopic MSD merging features, such as terraces, plastic ridges, shear sliding surfaces, cliffs, plateaus, and lowlands. Differences in fracture merging and growth features enable us to distinguish MSD component cracks. Quantitative fractography analysis revealed that the crack initiation occurred 1126 test cycles earlier, and that the overall crack propagation life slightly surpassed 1646 cycles as new cracks merged and joined. Diverse crack growth durations and rates across 11 fracture paths indicate asynchronous initiations and confirm interactions among component cracks. Cracks initiate flatter near the central crack colony but appear bumpier and more pronounced plastic deformation at the periphery. The overall MSD fracture evolution process was backtracked through synergetic fracture assessments, detailing the merging process of varied component crack colonies, enhancing our understanding of the spatial–temporal evolvement of cracking scenarios in actual aero-engine impellers.